Image sensor arrays typically comprise a linear array of photosensors which raster scan an image-bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors.
Although most scanning systems currently in use are ultimately digital systems, the “raw signal” coming out of the photosensors during the scanning process is an analog video signal, with the voltage magnitude corresponding to the intensity of light impinging on the photosensor at a given time. Thus, when signals are read out from the photosensors on a chip to be converted to digital data, different video levels, corresponding to the brightness of the reflected area being scanned by a particular photosensor at a particular moment, are output as a series of analog voltage levels.
In order to increase the readout speed of image signals from, for example, a linear array of photosensors, it is known to provide separate “odd” and “even” channels for the output of image signals. A basic example of this technique is shown in U.S. Pat. No. 5,638,121. In brief, alternate photosensors along a linear array respectively output image signals into separate odd and even video lines, and these video lines are subsequently multiplexed, thus yielding a single video stream.
FIG. 1 is a schematic view showing the basic elements of a readout system according to a prior-art implementation, illustrating an “odd-even” readout principle. There is provided on, for instance, a photosensor chip, a set of photosensors 10a-10z, which are connected by respective transistor switches 14a, 14b, etc. The switches in turn are activated by a shift register 18, which includes a set of stages (or, more precisely in the embodiment, half-stages) 20a, 20b, etc., arranged along a single line 22, and activated by a pixel clock line 24. When a digital 1 is passed through the stages 20a, 20b, etc., signals held on the photosensors 10a . . . 10z are caused to be read out of the system in linear order, to form a line of useable image data, such as for a digital scanner or copier.
According to this FIG. 1 example, the photosensors 10a . . . 10z are arranged in an interleaved manner with odd and even subsets, with the odd subsets of photosensors such as 10a and 10c connected to an odd video line 12a, and the even photosensors such as 10b and 10d, connected to an even video line 12b. Video line 12a receives the video outputs only of the odd photosensors, and the even video line 12b receives the video outputs only of the even photosensors. Because both the odd and even photosensors are controlled by a single shift register 18, having half-stages 20a, 20b, etc., the video voltage signals from a set of odd and even pixels can together be read out onto the odd and even video lines at a considerably faster rate than in a situation where all of the photosensors are reading out to a single video line. Another practical advantage is that, because fewer transistors are connected to each of the odd and even lines, there is less capacitance on each line than if both odd and even signal trains were read out on one line, and each video signal can settle to its final value faster.
With any sophisticated system for reading out images signals from a series of photosensors, a common practical problem is known as “dark non-uniformity” (DNU) or “fixed-pattern noise.” With each individual photosensor for an associated transfer circuit, there is likely to be a single dedicated amplifier (a “pixel amplifier”). Given the practicalities of constructing photosensors and circuits on a chip, it is likely that certain amplifiers, associated with certain photosensors, will consistently have higher output relative to other amplifiers associated with other photosensors. DNU is defined as the maximum difference in output voltage between any two pixels of an image sensor while in the dark. There exist basic techniques for overcoming DNU, such as mentioned in U.S. Pat. No. 5,654,755.
“Double sampling” is a technique that can be used to reduce the DNU contribution of the pixel amplifier. With this concept, the output of each pixel amplifier is sampled twice, once with the optical signal from the photosensor such as 10a and once with a common reference signal, so that the output signal from the pixel is defined as the difference between the two samples. Additional signal processing stitches the video back together and restores the output level. If the pixel amplifier offset is constant, the subtraction of the double samples, to the first order, eliminates its contribution to DNU. However, the problem with doing double sampling in a standard architecture is that the pixel amplifier is read out twice to the same video line, which effectively reduces the output data rate by 50%.
The present disclosure relates to a photosensor circuit architecture that enables double sampling of video outputs without a necessary decrease in output data rate.